RNA polymerase (RNAP) is ADP-ribosylated by the TY phage-induced Mod protein early in infection. The role of this modification has not been understood, partially because of the heterogeneity of RNAP purified from T4 infected cells. RNAP was reconstituted from ADP-ribosylated a and wild type b, b1 and subunits. The contamination of wild-type a was determined by mass spectrometry.